I watched this yesterday. The title and premise were vaguely familiar, but it wasn't until the first post-title scene (yes, that one) that I knew that I'd seen it before. I have a recollection of seeing it 30 years ago and maybe again 15-20 years ago. It was kind of weird watching it now, with coronavirus keeping everyone home and away from public places. On top of that, I just happened to watch it while this site was down, which was creepy. The character turning on the radio and receiving nothing but static was like accessing this site and getting an error message. It might as well be the end of the world for me if I can't access HF. Anyways, I quite enjoyed the first third. After that, it gets less interesting. It's never "bad," just a little disappointing that it doesn't maintain its early strength. It could've been a
classic sci-fi film if it had. I'd probably give the first third a 7/10 and the rest 5/10. I'm not sure if that warrants a 6/10, so I'd probably go with 5/10, like you. I'm glad that I watched it again, though, especially when I did.
The last half isn't as good as the first half, but it does include this epic one-liner...
Joanne: I wouldn't ride with you if you were the last man on Earth.
Api: [Puts on shades] I'm working on it.
Edit: I just read a great review that sums up my thoughts (and even ends with the same quote, hah):
The Quiet Earth (1985)